Histopathology of ultrashort pulse laser retinal lesions
Purpose: We wish to identify the pathologic effects of visible ultrashort laser pulses directed to the retina to assist in developing safety standards for such lasers. Methods: Picosecond and femtosecond laser energy was delivered to the retina of the Dutch Belted rabbit and the Rhesus monkey using previously described techniques (1,2). One hour and one day retinal laser lesion morphology was evaluated in serial step sections by light microscopy, and selected lesions were examined by transmission electron microscopy (TEM). Histopathology was compared to the corresponding fundus photographs and fluorescein angiograms. Results: Ultrashort laser pulses induced retinal damage with minimal associated choriocapillaris effect. At low energy (0.31 to 0.8 μJ at 90 fs, 0.6 to 2.0 μJ at 3 ps) we found RPE and outer retinal damage in a focal site ≤ 30 microns across. As energy increased (> 1.1 μJ at 90 fs, > 4 μJ at 5 ps) the lesions increased to -50 to 80 microns across and became full thickness often with inner retinal hemorrhage or rupture of the nerve fiber layer but with no corresponding increase in choroidal damage. With TEM ruptured melanosomes were seen centrally in the higher energy 90 fs lesions. Conclusions: Ultrashort pulses of laser energy in this model produce a novel retinal morphology which may have significant safety impact. The retinal effects may be explained by new models of damage mechanisms and of propagation of ultrashort laser pulses.